1. Field of the Invention
The invention relates to improved insulative coatings for electrical steels, and more particularly to phosphate based insulative coatings resistant to deterioration during heat treatments in reducing atmospheres.
2. Description of the Prior Art
As used herein and in the claims the terms "electrical steel" or "silicon steel" relates to an alloy the typical composition of which by weight percent falls within the following:
Carbon 0.060% maximum Silicon 2 - 4% Sulfur or Selenium 0.03% maximum Manganese 0.02 - 0.4% Aluminum 0.04% maximum Iron balance
At the present time, there is a great demand for silicon steels of sheet gauge for magnetic uses such as laminated cores for transformers and the like.
Magnetic cores, such as are used in power transformers and the like, require high interlamination resistivity. Much work has been done in this field to provide surface resistivity on electrical steels.
In the production of certain oriented grades of silicon steel a mill glass is formed during the high temperature anneal. Much work has been done toward the improvement of the insulative qualities of mill glass, as is exemplified in U.S. Pat. No. 2,385,332.
In some applications it is desirable to have an applied insulative coating rather than or in addition to the mill glass formed during the high temperature anneal. The need for an insulative coating which may be applied by the steel producer, so that the manufacturer may begin with sheets or coils having high surface resistivity, has led to the development of phosphate coatings. U.S. Pat. No. 2,501,846 and 2,492,095 are exemplary of such coatings. These phosphate coatings have been used with great success.
However, the manufacturer of electrical instrumentalities from silicon steel often requires that the silicon steel be subjected to a stress relief anneal or other heat treatment. Such stress relief anneals are frequently carried out at temperatures of from about 1450.degree.F. to about 1600.degree.F. (788.degree.C. to 871.degree.C.) in reducing atmospheres such as dry atmospheres containing 95% nitrogen and 5% hydrogen. Prior art phosphate coatings will demonstrate severe deterioration with respect to surface resistivity at temperatures as low as 1200.degree.F. (649.degree.C.) when the annealing atmosphere contains more than 2% hydrogen.
The present invention is based upon the discovery that phosphate based insulative coatings for electrical steels, with or without a mill glass base coating, will resist deterioration in reducing atmospheres if a mole ratio Mg/PO.sub.4 is maintained at from about 0.6 to about 1.5 and preferably from about 0.7 to about 1.3. The coatings of the present invention include one or more additional mineral acids so as to maintain the pH level in the range of from 0.5 to 3.0 and preferably from about 1 to about 2 for the final solution.